Carbon dioxide and other carbon oxides are frequently found in gas mixtures in industrial processes that contain other more industrially valuable gases, such as hydrogen or helium.
A major industrial process that is faced with gas mixtures containing more valuable hydrogen with less valuable carbon oxides is the steam-methane reforming industry and other forms of hydrocarbon reformations.
These industrial processes typically generate hydrogen rich synthesis gas containing less desirable carbon dioxide and carbon monoxide, as well as methane. This synthesis gas or reformate is purified by treatment in a pressure swing adsorptive separation to recover unadsorbed hydrogen and retain adsorbed carbon oxides. Examples of the prior art attempts to purify such gases are set forth below.
U.S. Pat. No. 3,176,444 relates to an improved process for purifying a fluid stream and also improving the recovery of the purified fluid. Process cycle steps including co-current depressurization, repressurization by feed and vacuum regeneration are all depicted. Purification of hydrogen from a variety of feed streams is taught in a number of examples. The '444 patent is of particular interest because it provides a list of suitable adsorbents for use in this process, including zeolite molecular sieves, activated carbon, silica gel and activated alumina. The patent clearly states that if the removal of carbon dioxide is desired, the correct choice of adsorbent is activated carbon because carbon dioxide is a strong adsorbate and carbon is a weak adsorbent. However, the important properties of the activated carbon which would improve hydrogen recovery and productivity are not addressed.
U.S. Pat. No. 3,252,268 teaches a specific combination of adsorption zones or beds operated in a specific manner to produce both a high purity product (preferably hydrogen) and a moderately pure product with improved product recovery. In processing tail gas from a hydroforming reaction, containing C.sub.1 -C.sub.4 hydrocarbons, hydrogen recovery ranging from 70-85% was achieved. The preferred adsorbent for the main beds is charcoal. A table in the patent presents operating, preferred and especially preferred charcoal characteristics as follows: 20-200 .ANG. pore diameter, 20-60 .ANG. pore diameter and 20-40 .ANG. pore diameter, respectively. The carbon used in the preferred unit was Columbia Grade ACC, 6.times.14 mesh (1,100 m.sup.2 /g). The bulk density of the material determined from the weight of carbon in the unit and the volume it contained (2,460 lbs/98 ft.sup.3) was 0.40 g/cc (25.1 lbs/ft.sup.3).
U.S. Pat. No. 3,323,288 details an improved pressure swing adsorption system in which detrimental heat effects are avoided by using adsorbent beds which are thermally integrated in the form of a packed bed heat exchanger. The separation of bulk CO.sub.2 from H.sub.2 is taught in one example in which 6.times.8 mesh activated carbon is used as the adsorbent. The bulk density of the adsorbent is 0.48 g/cc (30 lbs/ft.sup.3).
Another key patent in the area of hydrogen PSA process technology is U.S. Pat. No. 3,430,418. This invention relates to a process for separating gas mixtures including CO, CO.sub.2, CH.sub.4, N.sub.2, and H.sub.2 O from H.sub.2. In Example 1 of the patent, adsorbent columns layered with activated carbon and calcium A zeolite were used to produce a high purity hydrogen product (99.9999%). The process steps consisted of adsorption, pressure equalization, co-current depressurization, countercurrent depressurization, purge, and repressurization. The hydrogen recovery was 76.5%. The example notes that the carbon selectively removed water and CO.sub.2. Of special interest is the bulk density of the carbon adsorbent determined from the weight of the carbon and its contained volume (1,470 lbs/51.8 ft.sup.3) of 0.45 g/cc (28.4 lbs/ft.sup.3).
The physical properties of activated carbon used to purify hydrogen are also given in U.S. Pat. No. 4,077,780. This patent teaches a PSA process for separating gas mixtures containing ammonia, argon, methane, nitrogen, and hydrogen to recover both nitrogen and hydrogen. The adsorbent of choice for the recovery of hydrogen has a surface area in the range of 1,050 to 1,150 m.sup.2 /g, a particle diameter of 0.0075 feet, and a bulk density of 0.51-0.53 g/cc (32-33 lbs/ft.sup.3).
U.S. Pat. No. 4,853,004 describes a pressure swing adsorptive separation of gas mixtures, such as air or hydrogen and carbon dioxide, using zeolites and activated carbon using a composite of large and smaller particles of adsorbent. The examples separate air with activated carbon having a density of 0.664 g/ml (41.5 lbs./ft.sup.3) and 0.627 g/ml (39.14 lbs./ft.sup.3).
Thus, the preferred activated carbon adsorbents taught in the prior art have bulk densities ranging from 0.40 to 0.53 g/cc (25 to 33 lbs/ft.sup.3) or 0.62 to 0.66 g/cc (39 to 41.5 lbs./ft.sup.3).
The prior art has suggested the use of activated carbons for various separations including the separation of carbon dioxide from hydrogen, but the prior art has not addressed nor solved the problem of reducing gas generator sizing, such as steam-methane reformer size or the size of pressure swing adsorption systems to resolve carbon dioxide containing gas mixtures such as reformate from a steam-methane reformer. The present invention overcomes the problems and inefficiencies of the prior art by using activated carbon adsorbents having unexpected performance capability, unrecognized by the prior art, which provides opportunities for capital cost reductions, and efficiencies in product gas production, such as hydrogen product, which in turn allows gas utilizations in applications where previously such applications were unacceptable due to equipment size or cost constraints, particularly for small scale gas requirements where sizing and costs dictate applicability. These advantages of the present invention will be set forth in greater detail below.